In recent years, use of femto base stations is being expanded for such purposes as offloading the traffic from macro base stations placed outdoors and providing high-speed wireless environments in indoor areas. The femto base stations generally have smaller size and a lower output power than the macro base stations. The femto base stations have an access area (hereinafter referred to as “coverage”) of several meters to a few dozen meters. The femto base stations include closed subscriber group (CSG) femto base stations which are placed in general residences and operative in a closed access mode wherein access is limited to specific members. The femto base stations also include Non-CSG femto base stations which are placed inside stores and stations and operative in an open access mode which allows access of any users contracted with the operator of the femto base station.
In the latter Non-CSG femto base stations, a transmission signal transmitted from the indoors may pass through the outer walls and/or windows and leak to the outdoors. In that case, coverage may also be formed in the outdoors, so that outdoor terminals can connect to the femto base stations just like the indoor terminals. When the outdoor terminals connect to the femto base station, a channel capacity and/or a communication rate of the indoor terminals which are the main target of service are deteriorated thereby. When the outdoor terminals pass a femtocell, a handover from adjacent cells to the femtocell and a handover from the femtocell to adjacent cells occur in a short period of time, which causes an increase in signaling load to core networks.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-231236, and Patent Document 2: Japanese Laid-open Patent Publication No. 2013-197969 are introduced as the Related Art Documents
One method to solve the above-stated problem is to perform coverage control. In the coverage control, for example, the femto base stations lower transmission power so that handover frequency within a predetermined time period becomes a specified value or less, and thereby reduce leakage of radio wave to the outdoors. However, unlike the CSG femto base stations, the Non-CSG femto base stations do not identify members. Accordingly, it is difficult for the base stations to determine which terminal made a handover request to their own cells, an indoor terminal or an outdoor terminal. Accordingly, the femto base stations perform coverage control by using the handover frequency of all the terminals that come in and out the coverage of their own cells as an index.
Generally, when the femto base stations lower transmission power, the handover frequency of the outdoor terminals decreases. At the same time, their indoor coverage also decreases, so that indoor handover frequency starts to increase. However, the Non-CSG femto base stations do not distinguish outdoor terminals and indoor terminals. Accordingly, even when the Non-CSG femto base stations observe the handover frequency of all the terminals that come in and out the coverage of their own stations, the details of the handover frequency broken down to handover frequency of the indoor terminals and handover frequency of the outdoor terminals is not available. In the method of simply controlling the handover frequency of all the terminals to be a specified value or less without discriminating indoor and outdoor locations of users, it is difficult for the femto base stations to perform adjustment such as preferential reduction in handover frequency of the outdoor terminals. This makes it hard to perform appropriate coverage control.
FIG. 16 is an explanatory view of the problem in the related technology. In FIG. 16, x-axis represents transmission power P of a femto base station, and y-axis represents a handover frequency H. A solid line L11 expresses change in the handover frequency H of outdoor terminals. A dashed line L12 expresses change in the handover frequency H of indoor terminals. A dashed dotted line L13 expresses change in the handover frequency H of all the terminals. As illustrated in FIG. 16, in order to make the handover frequency H of the outdoor terminals equal to a handover frequency HT that is a target value, the femto base station may set the transmission power P to an ideal value Pideal. In the related control which uses the handover frequency of all the terminals as an index, the femto base stations can minimize the handover frequency of all the terminals. However, since the details of the handover frequency broken down to the indoor and outdoor frequencies are not available, the femto base stations sometimes set a value inadequate as the transmission power P from a perspective of the handover frequency of the indoor terminals. For example, the femto base stations may set the transmission power P to PCOV which excessively suppresses the handover frequency of the outdoor terminals. As a result, the handover frequency H of the indoor terminals expressed by the dashed line L2 may increase in contrast.